In most communication systems, increasing bandwidth translates into a greater transmission capacity. However, the physical medium or communication channel usually has some characteristics that vary in frequency, and only certain fragments or frequency bands can be used. For example, in an electrical grid, the power that can be injected starting from 30 MHz may vary in comparison to the power that can be injected at frequencies lower than 30 MHz. Also, an electrical grid has other characteristics that make an electrical grid less desirable as a transmission medium. For example, in an electrical grid, the noise floor decreases with the frequency, the attenuation increases with the frequency, and there is significant noise in the frequency-modulation (FM) radio band (from 88.5 to 108 MHz). Thus, there is a need to avoid certain parts of the frequency spectrum.
In the field of wireless communications, there is also a need to avoid certain parts of the frequency spectrum. In the context of Wireless Local Area Networks (WLAN), work has begun on a two new standards, IEEE 802.11ah and IEEE 802.11af, each of which will specify wireless network operation in sub-1 GHz frequencies. Low frequency communication channels are generally characterized by better propagation qualities and extended propagation ranges compared to transmission at higher frequencies. In the past, sub-1 GHz ranges have not been utilized for wireless communication networks because such frequencies were reserved for other applications (e.g., licensed TV frequency bands, radio frequency band, etc.). There are few frequency bands in the sub-1 GHz range that remain unlicensed, with different specific unlicensed frequencies in different geographical regions. The IEEE 802.11 ah Standard will specify wireless operation in available unlicensed sub-1 GHz frequency bands. The IEEE 802.11af Standard will specify wireless operation in TV White Space (TVWS), i.e., unused TV channels in sub-1 GHz frequency bands.
Thus, in both wired and wireless communication systems, it is necessary to configure the system to avoid certain bands of frequencies (e.g., a frequency band having a notch) so as to avoid interfering with other existing communication systems or to avoid frequencies at which the medium displays undesirable characteristics.